1. Field of the Invention
The present invention relates to well packer apparatus and methods for sealing tubing strings in wells as customarily used in the production of petroleum effluents from wells. The packer is used to seal the annulus between the well casing and the production tubing string through which the well effluents flow to the surface. Such packers are sometimes employed to provide seals for injecting fluids into the well and for a variety of other purposes.
2. Description of the Prior Art
Well packers have been developed to seal a conduit, or tubing string, to a surrounding well casing. Such packers are well known in the art for a variety of uses, including establishing a production tubing string within a well. The packer may be constructed to be anchored to the well casing in addition to supplying the necessary sealing means.
The packer is run in the well, or lowered, to a desired location, connected to a tubing string in an unset configuration, that is, exhibiting no sealing or anchoring engagement with the surrounding well casing. Then,the packer apparatus may be operated to lock the packer in place, and to provide a fluid-tight seal between the well casing and the packer itself. Such a setting procedure may be carried out in a variety of ways, the choice of which is related to the construction of the packer apparatus. In particular, weight-set packers employ the weight of the tubing string above the packer, when the packer has been lowered to the desired setting location, to manipulate the packer elements to achieve the set condition. The packer may be anchored to the well casing by effecting a radial expansion of an array of slips, which are pressed against the casing wall. In conjunction with the expansion of the slip array, resilient seals are forced into position between the body of the packer and the well casing. The packer must be fixed in this set configuration to prevent pressures within the well from moving the packer vertically, or otherwise disturbing the sealing and/or anchoring engagement between the packer and the well casing.
It may be necessary to subsequently re-set the packer at another location within the well, or to remove the packer from the well completely. Thus, it is particularly advantageous to construct the packer so that it is retrievable. Consequently, the packer must be able to be released from the set configuration, and returned to the configuration in which it was originally run in the well. Thus, the seals and the anchoring slips must be retractable from engagement with the well casing and the packer once again able to be moved by, say, manipulation of a tubing string.
Once the packer is set, and without disturbing its set condition, it may be necessary to move fluids beyond the packer in the annular region between the tubing string, which the packer seals to the well casing, and the well casing itself. Thus, a bypass may be provided by the packer apparatus, and which may be selectively opened or closed. Consequently, additional fluid-tight sealing must be provided to close off the bypass from fluid flow when desired.
Weight set packers constructed to provide the aforementioned features may be manipulated by longitudinal movement of the tubing string, or rotational movement of the tubing string, or a combination. Examples of packers involving such movement may be found, for instance, in U.S. Pat. Nos. 3,279,542; 3,357,489; and 3,420,306 to C. C. Brown, and No. 3,467,184 to Young.
Packers set in a well casing may be subject to large pressure differentials across the set seals due to the condition of the fluid in the annular region between the tubing string and the well casing. Such pressure differentials may, in general, occur with the resulting net force acting on the packer either from above or below the seals. Each of the aforementioned patents discloses one form or another of setting means by which the anchoring slips are urged outwardly against the well casing by wedging action from both vertical directions. That wedging action, when appropriately linked to the portions of the packer body experiencing the pressure differentials tending to move the packer, may be employed to resist the net forces resulting from the pressure differentials. Such packer construction is particularly shown in U.S. patent application Ser. No. 612,226, filed Sept. 10, 1975.
Similarly, the same type of pressure differentials in the annular region between the tubing string and the well casing may also tend to disturb the setting of the tubing string in relation to the opening or closing of the bypass. Consequently, it is desirable to provide means by which the seals which close off the bypass may be retained in sealing position in the face of such pressure differentials. The aforementioned U.S. patent application Ser. No. 612,226 filed Sept. 10, 1975 discloses a pressure compensation means which employs a split ring and piston arrangement responsive to the pressure differential tending to unseat the bypass seal. The pressure differential forces the piston to bear against the split ring to ultimately hold the bypass seal in position to close off the bypass. U.S. Pat. No. 3,659,647 shows a plurality of radially movable pistons acting in conjunction with a split, annular snap ring to maintain a packer in set position rather than to maintain a bypass closed. In that case, a pressure differential across the packer seals, with excess pressure from below the seals, acts on the pistons to maintain the mandrel passing through the packer body fixed relative to the set packer body. Neutralization, or reversal, of the pressure differential, or rotation of the tubing string relative to the set packer, permit the release of the packer from its set condition. A modified form of the mandrel hold-down assembly is shown in the same patent. An annular, axially movable piston cooperates with an annular, split snap ring to react to the same pressure differential, with increased pressure below the set packer, to drive the snap ring into tight, threaded engagement with the mandrel passing through the packer body. The snap ring, in turn, is limited in its vertical movement by an element of the set packer body. To release the mandrel, and the tubing attached thereto, for vertical movement, the pressure differential may be reversed, or the mandrel may be unthreaded from the snap ring.
It will be appreciated that, in a packer of a type wherein the anchoring slips as well as the sealing means are both set by relative movements of various packer elements, the maintenance of both the anchoring and sealing means in setting configuration may be critical. Thus, for example, once set in sealing engagement with the well casing, the sealing means of the packer may not be able to maintain a sufficiently fluid-tight seal unless the mechanism which forces the seal into sealing engagement with the well casing is itself held tightly enough in set configuration. In the case of a weight set packer, these setting forces, as well as those which maintain the anchoring slips in set configuration, may depend entirely on the weight of the tubing string by which the packer is lowered into position in the well casing. Thus, since the amount of this tubing string employed depends upon the depth to which the packer is lowered for setting, the setting forces themselves may then depend upon the location of the packer in the well. Hence, in relatively shallow wells, the weight of the tubing string may be inadequate to set the packer with sufficient forces.